If l is the distance along the tangent and R is the radius of the earth then the drop of the surface of the earth increases in a non linear way with distance and is found by the formula
d = R-(Rcos(asin(l/R)))Herewith a convenient table
Distance Angle Drop
30 0.499243 0.130702
60 0.998525 0.52284
90 1.497882 1.176501
120 1.997353 2.091835
150 2.496976 3.269051
180 2.996789 4.708418
210 3.49683 6.410266
240 3.997138 8.374984
270 4.497752 10.60302
300 4.99871 13.0949
330 5.500051 15.85119
360 6.001816 18.87252
390 6.504042 22.15961
420 7.006771 25.71321
450 7.510043 29.53417
480 8.013898 33.62337
510 8.518378 37.98179
540 9.023524 42.61046
570 9.529378 47.51049
600 10.03598 52.68305